Suppose that a light flash is emitted from the origin at the instant that points O and O' cross each other. If, as Einstein suggested, the velocity of light appears to be the same in all directions to any given observer, observers stationed at the origin in each frame will contend that, at later instants of time, the leading edge of the light pulse forms a sphere centered at the origin of the frame and that the observer in the other will have moved relative to the center of the sphere. Each observer will insist on their own contention and deny the argument of the other; there is no possibility of agreement between the two.
Sketch two separate pictures, each one illustrating the claim of one of the two observers. Note the symmetry or "reciprocity" of their points of view. I don’t know how to sketch these, where to start and what it would look like.